1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image by pinching and advancing a recording material bearing said toner image with a pair of rotary members, and more particularly to a fixing device provided with means for applying releasing agent onto said rotary members.
2. Related Background Art
Among the fixing devices for fixing toner image, there is being widely employed a heat roller fixing device in which the recording material is pinched and transported by a heat roller heated by a heater and a pressure roller pressed to said heat roller.
An example of such heat roller fixing device is shown in FIG. 4.
A recording material T bearing toner image thereon enters a nip 38 between a fixing roller 21 and a pressure roller 25, and the toner image is fixed to said recording material by heat and pressure.
For preventing offsetting of the toner, there is provided a releasing agent applicator 32 in a predetermined position of the fixing device 17. In said applicator 32, silicone oil 33 (for example dimethyl silicone oil KF96300CS manufactured by Shinetsu Chemical Co., Ltd.) contained in an oil tank 32a is picked up by rollers 34, 35, limited to a predetermined amount by a regulating blade 40, and applied onto the fixing roller 33.
Also in order to avoid wasted consumption of the silicone oil, it is also known to effect on-off contact between the oil applying roller 35 and the fixing roller 21.
FIG. 4 illustrates the application area of the silicone oil in case of on-off contact of the oil applying roller 35. As will be understood from FIG. 4, the oil 33 is applied earlier than the leading end of the recording material T, so that the oil reaches nip 38 prior to the entry of the recording material into said nip.
The recording material is generally composed of paper, but resinous films are being used more widely for meeting various copying requirements. Most well-known is overhead projector film, or so-called transparency film, but recording films are used for other various purposes.
However such film-shaped or resinous recording materials, having smoother surfaces than paper, pose difficulty in entering the nip between the fixing roller 21 and the pressure roller 25. Thus the recording material stops advancing through said nip upon contacting said nip of said rollers, eventually resulting in sheet jamming. Particularly, the presence of early applied oil in the nip causes the slippage of film with a smooth surface, thus enhancing the difficulty of entry into the nip.
Also such film, even if thermally resistant, may be softened or become undulated before entering the nip due to the heat received from the fixing roller or the pressure roller. Such fact also aggravates the difficulty of entry into the nip.
Use of such film is also known to require the reduction of the fixing roller speed, thereby effecting slower image fixation for the resinous recording material or the like requiring sufficient image fixation. For example, the speed of the fixing roller is reduced, from 90 mm/sec for paper, to 20 mm/sec for a resinous recording material. Such lower speed is employed in the case of a transparency film, for increasing the optical transmittance of toner and is employed in the case of fixing plural toner layers onto the resinous recording material as in a color image forming apparatus for achieving sufficient fixation even in the lowermost toner layer that is most difficult to fix. However such low-speed rotation of the fixing roller also increases the difficulty of entry of the recording material into the nip, because of the presence of a larger amount of oil at the entrance side of the nip 38 due to the low-speed rotation of the fixing roller and because of the enhanced softening and undulation of the recording material resulting from the prolonged exposure of the recording material to the heat.